docs: fix document about camera distortion parameters

[kamino410]

This pullrequest changes

The current documentation says that positive k1 causes barrel distortion and negative k1 causes pincushion distortion.
But it seems the opposite.

According to the formula, distorted point (x'', y'') moves toward the outside of the image when k1 > 0.
So it should cause not barrel distortion but pincushion distortion.

[catree]

According to the formula, distorted point (x'', y'') moves toward the outside of the image when k1 > 0.

I agree with that.

So it should cause not barrel distortion but pincushion distortion.

Barrel and pincushion distortion (from Matlab documentation):

[kamino410]

That's true.
But Edmund Optics defines them as follows.

At least, I'm sure that cameras whose k1 is negative take figures having barrel distortion because

• I tested leftXX.jpgs in https://github.com/opencv/opencv/tree/master/samples/data and got -0.27437524 | -0.03096935 | 0.00089529 | -0.00022190 | 0.27721061 as distortion params.
• undistort() returns an image having pincushion distortion with k1 < 0.

[paroj]

generated with k1=2, rest zero

[catree]

@paroj Thanks for taking the time to check this.